A perfluorocarbonsulfonic acid membrane has been one of the electrolyte membranes used in the conventional solid polymer electrolyte membrane fuel cells (patent document 1). However, the perfluorocarbonsulfonic acid membrane, to its disadvantage, cannot be used in a dry condition as water contained in the membrane forms a proton-conducting path (non-patent document 1). Various membranes including the following have been tested for their capability of enhancing the proton conductivity in a dry condition: silica dispersion perfluorosulfonic acid membranes (patent document 2); inorganic-organic composite membranes (patent document 3); and phosphoric acid-doped graft membranes (patent document 4). However, all of the aforementioned membranes essentially require the presence of water in them for manifestation of proton conductivity and they have not offered a substantial solution to the problem of providing proton conductivity without humidification.
Thus, there is a growing demand for the development of proton conductors that do not require water and one of the proposed approaches is utilization of ionic liquids. The “ionic liquid” is a general term used for compounds that consist of a combination of an anion and a cation and melt below 100° C. It is advocated that an ionic liquid with a combination of ions tailored to a particular application can manifest the required properties (non-patent document 1). The applications proposed for ionic liquids include reaction solvents, electrolytes for batteries, lubricants, and heat transfer media.
Several proposals have been made on proton exchange materials mainly intended for use in fuel cells: for example, the use of aprotic ionic liquids composed of nitrogen-containing quaternary salts such as quaternary ammonium salts, quaternary pyridinium salts, and quaternary imidazolium salts and polymer materials having ion exchange groups in proton exchange membranes (patent documents 5-7) and the use of protic ionic liquids composed of imidazole compounds in proton exchange liquids and membranes (patent documents 8-13). However, the proton conductivity cited in the examples was in the range of 10−4 to 10−3 S/cm at most without humidification and this necessitated a further improvement for utilization as a proton exchange membrane without humidification. A proton exchange liquid or membrane obtained by using an imidazole-based ionic liquid containing a hydrogen fluoride-based compound as an anion is proposed as a material of high proton conductivity; however, the use of this material would pose a problem in the treatment of hydrogen fluoride.
Several proposals have also been made on proton-conducting solid electrolytes mainly intended for use in fuel cells: for example, the use of aprotic ionic liquids composed of nitrogen-containing quaternary salts such as quaternary ammonium salts, quaternary pyridinium salts, and quaternary imidazolium salts and polymer materials having ion exchange groups in proton exchange membranes (patent documents 5-6) and the use of protic ionic liquids composed of imidazole compounds in proton exchange liquids and membranes (patent documents 7-12). However, the proton conductivity of these materials shown in the examples was in the range of 10−4 to 10−3 S/cm without humidification and this necessitated a further improvement for utilization as a proton exchange material without humidification. Furthermore, all of the reported materials are prepared by impregnating a polymer material with an ionic liquid and a problem latent in these materials is leakage of the ionic liquid. To solve this problem, studies are in progress to develop solid electrolytes by increasing the molar mass of an ionic liquid. For example, the polymerization of an N-vinylimidazole salt, which is a protic ionic liquid, is known (patent document 13); however, the proton conductivity of the resulting polymer was not at a satisfactory level and needed to be improved further.
In spite of the advocacy described earlier that an ionic liquid with a combination of ions tailored to a particular application can manifest the required properties, quaternary imidazolium ions, alicyclic quaternary ammonium ions, quaternary alkylammonium ions, and the like are used in the majority of developmental works currently underway and it has been considered necessary to search for ions of a novel skeleton to make a breakthrough.
Accordingly, the inventors of this invention have made a search for novel skeletons in their studies aimed at ion-conducting materials and noted a singularity of chemical structure possessed by melamine compounds, namely, 1) a non-localized structure of electrons and 2) a high content of atoms capable of capturing protons, typically nitrogen atoms, in the skeleton. However, melamine compounds have been used so far as raw materials of pharmaceuticals and melamine resins and as additives to polymer materials and their use as an ion-conducting material has been known only in the patent document 15. This document discloses the proton conductivity of a mixture of unsubstituted melamine, cyanuric chloride, and p-toluenesulfonic acid, but it did not reach a practically useful level.    Patent document 1: JP7-90111 A    Patent document 2: JP6-111827 A    Patent document 3: JP2000-90946 A    Patent document 4: JP2001-213987 A    Patent document 5: JP2003-257484 A    Patent document 6: JP2004-31307 A    Patent document 7: JP2004-311212 A    Patent document 8: JP2006-32181 A    Patent document 9: JP-2006-32213 A    Patent document 10: JP2005-44550 A    Patent document 11: JP2005-44548 A    Patent document 12: WO2003-083981 A    Patent document 13: JP2005-174911 A    Patent document 14: JP2005-251466 A    Patent document 15: JP6-145642 A    Non-patent document 1: Chem. & Eng. News, May 15, 2000